A liquid crystal display device is a flat display device having excellent properties such as high definition, a flat shape, light weight, and low power consumption. Recently, due to an increase in display ability, an increase in production ability, and an increase in price competitiveness against other display devices, the market of the liquid crystal display device has spread rapidly.
An in-plane switching mode (IPS mode, see Patent Literature 1) and a multi-domain vertical aligned mode (MVA mode, see Patent Literature 2) in particular are applied to liquid crystal televisions as a liquid crystal display device of a wide viewing angle which is free from a problem such as a great decrease in a display contrast ratio and inversion of display gradations when a display surface is seen from a skew direction.
Although display quality of a liquid crystal display device has been improved, there appears a new problem of viewing angle dependency: a problem of difference in gamma characteristic when seen from a front and gamma characteristic when seen from a skew direction, i.e. a problem of viewing angle dependency in gamma characteristic. Gamma characteristic here indicates dependency of display luminance on gradations, and gamma characteristic being different between when seen from a front and when seen from a skew direction indicates that the state of gradation display varies depending on a direction in which the display surface is seen. This is problematic particularly when displaying an image such as photograph and when displaying television broadcasting etc.
The viewing angle dependency of gamma characteristic is more evident in the MVA mode than in the IPS mode. On the other hand, it is more difficult to produce a liquid crystal panel of the IPS mode with high contrast ratio when seen from the front than to produce a liquid crystal panel of the MVA mode with high contrast ratio when seen from the front. In view of the above, it is desirable to improve viewing angle dependency of gamma characteristic in the liquid crystal display device of the MVA mode in particular.
With respect to this problem, Patent Literature 3 discloses a liquid crystal display device and a driving method thereof, each capable of improving viewing angle dependency in gamma characteristic, excess brightness characteristic in particular, by separating one pixel into a plurality of sub-pixels with different brightness. Such display or driving is referred to as area coverage modulation display, area coverage modulation drive, multi-pixel display, or multi-pixel drive.
To be specific, an auxiliary capacitor (Cs) is provided for each of a plurality of sub-pixels (SP) in one pixel (P), and an auxiliary capacitor counter electrode (connected with a CS bus line) constituting the auxiliary capacitor is electrically independent with respect to each sub-pixel. By changing a voltage to be supplied to the auxiliary capacitor counter electrode (the voltage may be referred to as an auxiliary capacitor counter voltage, an auxiliary capacitor signal voltage, an auxiliary capacitor signal, or a CS signal), effective voltages applied on individual liquid crystal layers of the plurality of sub-pixels are made different with use of a capacitive divider.
However, if the multi-pixel structure described in Patent Literature 3 is applied to a liquid crystal television with high definition or with a large size, cycle of oscillation of an oscillating voltage gets shorter as a display panel has higher definition or larger size. This raises a problem such as difficulty in preparation of a circuit for generating an oscillating voltage, an increase in power consumption, greater influence of rounding of a waveform due to electric load impedance of a CS bus line. With respect to this problem, Patent Literature 4 discloses providing a plurality of CS main lines that are electrically independent from each other and connecting a plurality of CS bus lines with each of the CS main lines so as to lengthen a cycle of oscillation of an oscillating voltage to be applied to an auxiliary capacitor counter electrode via the CS bus line.
If a current voltage continues to be applied to a liquid crystal layer of such liquid crystal display device for a long time, elements get deteriorated. Therefore, in order to secure a long life of such liquid crystal display device, it is necessary to perform alternating driving (inversion driving) in which the polarity of a voltage to be applied is inverted periodically. However, in a case where an active matrix liquid crystal display device employs frame inversion driving in which the polarity of a voltage is inverted with respect to each frame, it is inevitable that some unbalance is seen in a plus/minus voltage to be applied to liquid crystal due to anisotropy of liquid crystal dielectric constant, variation in pixel potential that is caused by parasitic capacitance between a gate and a source of a pixel TFT, and a slip of a center value of a counter electrode signal. Consequently, a minor variation in luminance occurs at a frequency that is a half of a frame frequency, making a user see flickers. In order to solve this problem, there is generally employed inversion driving in which pixel signals have opposite polarities between adjacent lines or adjacent pixels as well as voltages are inverted with respect to each frame.
When dot inversion in which the polarity of a voltage is inverted with respect to each pixel is performed, a charging rate of a pixel drops due to signal delay in a data signal line. In order to solve this problem, there is proposed a technique for inverting the polarity of a data signal voltage with respect to a plurality of horizontal periods (a plurality of rows). However, this technique still raises a problem that a charging rate of a pixel drops at a row where the polarity of a data signal voltage is inverted.
In order to solve this problem, Patent Literature 5 discloses a technique in which a dummy horizontal period is provided after inversion of the polarity of a data signal and gate-on pulses whose pulse width corresponds to a plurality of horizontal periods are applied to all scanning signal lines in such a manner that the gate-on pulses have the same pulse width. FIG. 92 is a voltage waveform chart showing driving by this technique. In FIG. 92, (2) represents a latch pulse LP1, (3) represents image data D to be latched by a signal-side drive circuit and output to a signal line SL with respect to each horizontal scanning period, (4) represents a polarity signal P of an image signal voltage, and (5)-(12) represent scanning signal voltages of individual scanning lines. This technique improves display unevenness due to the difference in a charging property.
Further, Patent Literature 6 discloses a technique in which the width of a gate-on pulse after inversion of the polarity of a data signal is made larger than the width of a gate-on pulse with no inversion of the polarity of a data signal so as to increase a charging rate of a first row where the polarity of the data signal is inverted. FIG. 93 is a voltage waveform chart showing driving by this technique. FIG. 93 shows gate signals at 4ith to [4(i+1)+1]th rows and a data signal.